Gas generators serve in vehicles in particular for the activation of vehicle occupant restraint systems, such as for example an inflatable impact protection cushion or gas bag or a pneumatic knee guard, in the case of a vehicle accident. They usually contain a pyrotechnic propellant, which after its ignition releases a gas mixture which is under high pressure, and activates the safety arrangement.
On exceeding a particular ambient temperature, e.g. in the case of a vehicle fire, the gas generators can be triggered in an uncontrolled manner and therefore also endanger people. Furthermore, it is known that the pyrotechnic propellants which are usually used do not react normally at increased temperature, but rather react in an accordingly accelerated and violent manner, in unfavourable circumstances even in the manner of an explosion. The housings of the gas generators are not designed for such a violent reaction and in so doing can fragment. Gas generators having light metal housings, in addition suffer from a weakening of the housing structure at higher ambient temperature. This effect can also lead to the housing fragmenting on reaction of the propellant. Under these circumstances, an increased risk of injury exists for the vehicle occupants.
Gas generators equipped with a pyrotechnic propellant therefore contain a so-called “auto-ignition device”, which usually contains a primary explosive and is connected with a secondary explosive of the ignition device of the gas generator, or with the pyrotechnic propellant. The auto-ignition temperature of the primary explosive usually is in a temperature range of approximately 145 to less than 200 degrees C. As soon as the ambient temperature exceeds the auto-ignition temperature, the primary explosive of the auto-ignition device ignites and activates the secondary explosive or the pyrotechnic propellant. The propellant then reacts in a controlled manner, before the housing structure of the gas generator is weakened too greatly by the temperature rise or an undesirably violent reaction of the propellant can lead to a fragmenting of the gas generator.
An auto-ignition device of the type described above is known for example from the DE 38 24 469 C1. This auto-ignition arrangement comprises a metal housing incorporated into the outer wall of the gas generator housing, into which a primary explosive is introduced. The metal housing includes a window provided with a seal, with the window being directed to a secondary explosive of the ignition device or to the gas-generating propellant of the gas generator.
In the gas generator known from the U.S. Pat. No. 5,772,243 A, an auto-ignition charge, which spontaneously ignites at an ambient temperature of approximately 177 degrees C., is introduced into an annular groove in the igniter support. The annular groove, with the auto-ignition charge situated therein, is closed by a thin metal foil and is directed to the secondary explosive of the ignition device of the gas generator.